The present invention relates to lithographic printing plate materials to which silver complex diffusion transfer process is applied.
A lithographic printing plate relates to an oleophilic image portion which is receptive to oily inks and an oil-repellent non-image portion which is not receptive to inks. In general, said non-image portion comprises a hydrophilic portion receptive to water. In usual lithographic printing, both water and ink are provided on the surface of lithographic printing plate and the image portion selectively receives a coloring ink and the non-image portion selectively receives water and the printing is carried out by transferring ink on the image portion onto a material to be printed, such as paper.
Therefore, in order to make good prints, a great difference between the oleophilicity of the image portion and the hydrophilicity of the non-image portion is required and when water and an ink are provided on the surface of the printing plate, the image portion receives a sufficient amount of ink and the non-image portion does not completely receive the ink.
Lithographic printing plates made by silver complex diffusion transfer process, especially, lithographic printing plates having a physical development nuclei layer on a silver halide emulsion layer are disclosed, for example, in U.S. Pat. Nos. 3,728,114, 4,134,769, 4,160,670, 4,336,321, 4,501,811, 4,510,228 and 4,621,041. The exposed silver halide crystal undergoes chemical development by DTR development to become black silver which serves as a hydrophilic nonimage portion. On the other hand, unexposed silver halide crystal is converted to a silver complex by a silver salt complexing agent in the developer and the complex diffuses to the surface physical development nuclei layer and physical development occurs due to the presence of nuclei to form an image portion mainly composed of ink-receptive physically developed silver.
In this way, the above-mentioned lithographic printing plate utilizes a silver image precipitated on the physical development nuclei layer provided on a gelatin-silver halide emulsion layer as an ink-receptive image portion and has the defects that its resistance against mechanical abrasion is insufficient as compared with general lithographic printing plates (such as PS plate) and the image portion is torn off or the ink-receptivity of the image portion is gradually lost.
If hardness of gelatin is enhanced or amount of the physical development nuclei is increased in order to overcome these defects, background areas are stained resulting in much decrease of printing endurance. Furthermore, staining of background occurs or ink-receptivity deteriorates during storage of long period of from production of lithographic printing plate materials to making of printing plates and improvements of these defects have been desired.